Poly(Styrene-b-Isobutylene-b-Styrene) (SIBS) is a thermoplastic elastomer that has gained attention recently due to its high degree of biocompatibility. Due to its biocompatibility, SIBS has been found to be useful for a variety of application, such as stent coating, stoppers, glaucoma shunt, and tubing. This linear block copolymer has a triblock structure formed by a polyisobutylene (PIB) core sandwiched between blocks of polystyrene (PS). The formulation of SIBS can be tailored for different applications by changing the weight percentage of PS or by changing the molecular weight of the polymer chains. The hard PS blocks provide SIBS with a glassy microstructure that enhances mechanical strength and rigidity of the material, while the PIB has a soft microstructure with increased chain mobility that gives the polymer its elastomeric properties. The possibility of tailoring mechanical properties, together with the high degree of biocompatibility, makes SIBS an ideal material for use in biomedical devices.
However, there is a high cost associated with making SIBS. The high cost (30-40%) of most SIBS products is largely due to the expensive bifunctional polymerization initiator need for synthesis. Typically, that expensive bifunctional polymerization initiator is 1-(tert-butyl)-3,5-bis(2-chloropropan-2-yl)benzene. A commercial version of SIBS, named SIBSTAR, available from Kaneka Co., used mainly as additive in various industrial applications, is strongly contaminated with ill-defined diblocks.
Block copolymers of similar compositions might have diverse mechanical properties due to their composite nature. Parameters such as molecular weight, block weight percentage, and polymer chain structure are known to give rise to different microstructures that in turn lead to different material properties. Different grades of SIBS can have very different morphologies based on the ratio of hard phase to soft phase. At lower contents of PS, the hard phase forms spherical domains through the soft matrix. As the PS content increases, the spherical domains become double gyroid structures, and as the PS content is further increased, the structure of the hard phase becomes lamellar. It is likely that the incompatibility of the soft and hard phases leads to micro-phase separations and results in the different morphologies described. It is well known that for composite systems, the interface between different phases plays a major role in the performance of the material. A weakened interface might lead to premature cracking and failure. Additionally, the method of fabrication for SIBS might play a very important role due to the incompatibility of the different phases. Therefore, different methods may result in different qualities of the interface.
However, for all of its attributes, SIBS has been found to be of modest strength and tends to exhibit high creep. Therefore, the need exists for a new material, useful for implantable medical devices and industrial applications, that has the key advantageous properties of SIBS, such as biocompatibility, biostability, elasticity, and processability, but that also exhibits higher strength, toughness, and diminished creep, which SIBS does not exhibit. Furthermore, this new material should be able to be synthesized without the use of a costly multi-functional initiator.